<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        header {
            width: 80%;
            height: 300px;
            margin: 100px auto;
            min-width: 800px;
            display: flex;
        }
        
        section:nth-child(1) {
            height: 100%;
            background: palegoldenrod;
            width: 200px;
        }
        
        section:nth-child(2) {
            height: 100%;
            background: palevioletred;
            /* width: 200px; */
            flex: 1;
        }
        
        section:nth-child(3) {
            height: 100%;
            background: palegreen;
            width: 200px;
        }
    </style>
</head>

<body>
    <header>
        <section></section>
        <section></section>
        <section></section>
    </header>
</body>

</html>